1. Field of the Invention
This invention pertains to audio amplification. Specifically, the present invention relates to a device and method for eliminating undesirable audio feedback which occurs when audible sound travels from a loudspeaker to a live or active microphone, when the loudspeaker is amplifying audio signals received at the active microphone.
2. State of the Art
Audio feedback is more than a nuisance in state of the art audio systems. This is because it can cost time and money to physically configure or reconfigure audio systems to reduce, but not eliminate, the cause of the problem. The audio systems which are vulnerable to audio feedback are those which include an active microphone and an amplification system including loudspeakers, such as a public address system. Audio feedback is a result of sound from the loudspeakers feeding back into the active microphone. The phenomenon is manifested as a high pitched electronic squeal which typically rises in volume until adjustments are made to the audio system to correct the problem. Corrective action typically includes turning down the volume so that the sound which is being propagated by the loudspeakers does not exceed an audio feedback threshold at the microphone. Once the audio feedback threshold is exceeded, a feedback loop arises which requires that corrective action be taken to break the loop.
FIG. 1 shows a typical state of the art audio system 60 as described above. The audio system 60 is shown having two loudspeakers 62 to provide capability for stereo broadcasting. However, only a single loudspeaker 62 is necessary for the audio feedback to occur. The other critical elements of the audio system 60 are the active microphone 64 and an amplification/processing system 66. It should be readily apparent that audio systems capable of live broadcasting of sound involving detection by a microphone can include many other system components. For example, the audio system 60 might be part of a karaoke system which mixes recorded music with live input from a microphone before the mixed sound is emitted from loudspeakers. What is important to recognize is that audio feedback is a result of an audio system having loudspeakers and an audio input, where the audio input picks up enough of the sound coming from the loudspeakers to cause audio feedback. This phenomenon is well understood by those skilled in the art of live audio systems.
To understand the present invention, it is also necessary to relate the present invention od the general acoustic speaker art and the ongoing effort to reproduce sound in its purest form. In an earlier patent application under Ser. No. 08/684,311 of the same inventor, a detailed background of prior art in speaker technology using conventional speakers having radiating elements was reviewed and is hereby incorporated by reference.
There are several disadvantages which are inherent in such conventional speakers. The primary disadvantage is distortion arising from the mass of the moving diaphragm or other radiating component. Related problems arise from distortion developed by mismatch of the radiator element across the spectrum of low, medium and high range frequencies-a problem partially solved by the use of combinations of woofers, midrange and tweeter speakers.
Attempts to reproduce sound without use of a moving diaphragm include technologies embodied in parametric speakers, acoustic heterodyning, beat frequency interference and other forms of modulation of multiple frequencies to generate a new frequency. In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range. Ideally, resulting compression waves would be projected within the air as a nonlinear medium, and would be heard as pure sound. Despite the ideal theory, general production of sound for practical applications has alluded the industry for over 100 years. Specifically, a basic parametric or heterodyne speaker has not been developed which can be applied in general applications in a manner such as conventional speaker systems. However, there are several patents and matters known to those skilled in the art which demonstrate attempts at creating a parametric speaker system.
For example, a publication by Robert T Beyer of Brown University in 1856 noted the research of H. von Helmholtz commenting on xe2x80x9ccombination tonesxe2x80x94that do not come directly from the sound sources but that arise secondarily through the interaction of the two primary tones.xe2x80x9d Ann. Phys. Chem. 99:497+(1856). These early observations noted that both sum and difference tones were developed, giving rise to theoretical questions of cause. This phenomenon, known in music as Tartini tones, was originally postulated to be a form of beat frequency arising as a difference frequency between two original audible frequencies. In the mid 1800""s Helmholtz discovered the presence of the xe2x80x9csumxe2x80x9d frequency, suggesting that the phenomenon might be nonlinear. Nevertheless, little progress developed with respect to sum and difference tones as an acoustic phenomenon until the early 1900""s.
In 1921, U.S. Pat. No. 1,616,639 disclosed an application of two sound waves of different frequencies simultaneously impressed on a vibrating body (the ear) to form new waves equal to the sum and difference of the two interacting frequencies. The perceived application of this system was as part of an auditorium speaker system as shown in prior art in FIG. 2. Specifically, the reference suggests that a carrier frequency xe2x80x9cCxe2x80x9d can be amplitude modulated with a sound signal xe2x80x9cSxe2x80x9d to generate two sideband frequencies C+S and Cxe2x88x92S. One of the sideband frequencies (i.e. Cxe2x88x92S) is then filtered out, with the remaining sideband (C+S) being transmitted toward an audience 2 from the front end of the auditorium. The carrier frequency C was transmitted from a separate speaker 24 at the opposite end of the auditorium. As these two opposing wave fronts arrive at an audience member, the modulated xe2x80x9cdifferencexe2x80x9d wave S is supposedly developed by the ear based on the concurrent imposition of the two high frequencies on the tympanic membrane.
In 1931, U.S. Pat. No. 1,951,669 was issued, teaching a similar concept based on the theory of heterodyning or interference of super-audible air waves. This early system is represented in prior art FIG. 3 and comprises two opposing loud speakers 46 and 47 at opposite ends of a room, each being operated at separate super-audible frequencies 44 and 45. By directing these speakers toward an intermediate region of air, an audible signal was to be formed, based on the difference between the interfering frequencies. However, those skilled in the art are unable to develop meaningful sound from such a configuration where opposing speakers are directed toward each other as illustrated in FIG. 2.
An additional line of thought developing this general theory of sound production is reflected in an article by Peter J. Westervelt, xe2x80x9cParametric Acoustic Arrayxe2x80x9d, published in The Journal of the Acoustical Society of America, Vol 35, No 4, April 1963. This disclosure, which is admittedly theoretical as opposed to experimental, attempts to mathematically define the requirements for generation of a difference wave as part of a parametric speaker system. After discounting earlier efforts to generate sound when opposing waves intersect at nonzero angles, the article relies on the assumption that if the two separate sound sources can generate two beams of sound which are (i) perfectly collimated, (ii) superimposed and (iii) with a beam of sound so narrow as to constitute a xe2x80x9clinexe2x80x9d along the axis of the primary beams, sound generation could be achieved. (Page 535, col 2) The system requires use of xe2x80x9ca microphone in the carrier beam, and the output of the microphone must then be fed into a conventional radio set in order to demodulate the signal.xe2x80x9d Id. P 537. Here again, no practical application of this theoretical study has developed, suggesting the absence of one or more key teachings. Neither is any reference made to feedback control or special characteristics of a parametric speaker system in this domain.
After this non-exhaustive but revealing history into the development of the parametric speaker, it is important to note that a commercial quality parametric speaker system which utilizes the phenomenon of acoustical heterodyning was created by the present inventor. In addition to the patent application of the present invention already mentioned above, other patent applications have also been filed by the inventor which teach further refinements of the technology. During the course of investigative research in this general field of parametric or acoustic heterodyning technology, the present inventor discovered an unanticipated property of parametric output which relates to the problems of audio feedback.
With this background as described above, it is observed that there is a serious present need for an audio system which is not vulnerable to the disruptive and costly effects of such audio feedback. It would be a significant advancement to have an audio system which provides sound reproduction without the frustrating potential of audio feedback within the system.
Accordingly, it is an object of the present invention to provide a method and apparatus for eliminating audio feedback from an audio system including an active audio input, an amplifier, and a speaker.
It is another object to provide a method and apparatus for eliminating audio feedback from an audio system which includes a parametric speaker.
It is another object to provide a method and apparatus for eliminating audio feedback from an audio system which includes heterodyne processing of audio input, and a conventional speaker for broadcasting the processed audio input.
The preferred embodiment of the present invention includes an active microphone coupled to an amplifier which transmits signals received at the active microphone to a parametric speaker for broadcasting. In the preferred embodiment, the principle of operation is based on eliminating audio feedback by utilizing a parametric speaker to broadcast the signals.
In another aspect of the invention, it is follows that a conventional speaker system can still be used and audio feedback still eliminated if processing of the signals from the active microphone occurs utilizing the same signal processing which occurs in the audio system which includes a parametric speaker.
These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings.